Absorption, Emission and Fluorescence Spectroscopies
Chem 151L. R. Corn
Review:
Light behaves like an electromagnetic wave...
2.998 x 108 meters/sec
λ = the Greek letter lambda
ν = the Greek letter nu
velocity = wavelength x frequency
meters/sec = (meters) x (sec)-1
The units (sec)-1 are called Hertz (Hz)!
Heinrich Hertz
1857-1894
Review:
But light also behaves like a particle!
We call the particles “photons”,
each with a energy given by:
h = Planck's constant
6.626 × 10−34 Joule sec
Max Planck
1858-1947
Review:
Atoms have quantized energy levels too, and can absorb
and emit photons of energy equal to the difference in
energy between two levels
∆E = hν
Grotrian diagram for Hydrogen
Review:
Molecules also have quantized energy levels, and can
absorb and emit photons of energy equal to the difference
in energy between two levels:
LUMO
hν
HOMO
M + hν → M*
∆E = hν
Absorption Spectroscopy measures the amount of light
absorbed by a sample by this process.
M + hν → M*
Transmittance: T = P/P0
Absorbance: A = log(P0/P)
For atoms: Atomic Absorption Spectroscopy
For molecules: Molecular Absorption Spectroscopy
The Absorption Spectrum plots the amount of light
absorbed by a sample as a function of photon wavelength.
Absorption spectrum
Beer’s Law relates the amount of light absorbed by a
sample to the concentration of the absorbing species.
Absorbance
A = log(P0/P)
Beer’s Law: A = εbc
b = pathlength (in cm)
c = concentration (in M)
ε = molar absorptivity (units of M-1cm-1)
Absorption versus Emission
Absorption is the process that consumes a photon and
puts the atom or molecule in an excited state.
M + hν → M*
Emission is the process that creates a photon and takes
the the atom or molecule in an excited state back to the
ground state.
M* → M + hν
The Emission Spectra of H, He and Hg.
Fluorescence Spectroscopy
Fluorescence is the process that first consumes a photon
and puts the atom or molecule in an excited state...
M + hν → M*
And then emits a photon of lower energy which takes the
the atom or molecule back to the ground state.
M* → M + hν’
hν > hν’
Fluorescence Spectroscopy
Net reaction:
M + hν → M* → M + hν’
hν > hν’
The emitted photon has less energy than the absorbed
photon because the molecule loses some energy (by
vibrating and rotating) in the excited state:
M*
M
hν’
The Fluorescence Spectrum plots the amount of light
absorbed by a sample as a function of photon wavelength.
P0
P
PF
P0 = power of incident light beam (units: W)
P = power of transmitted light beam.
PF = power of emitted fluorescence.
Quantitative Fluorescence Spectroscopy
The power of the emitted fluorescence is proportional to the
absorbed power, P0-P:
PF = ΦF (P0 − P)
where ΦF, the quantum yield, is:
€
photons emitted
ΦF =
photons absorbed
substituting from Beer’s law:
€
(
PF = ΦF P0 1− e
−εbC
)
Quantitative Fluorescence Spectroscopy
(
PF = ΦF P0 1− e
−εbC
)
the exponential here can be expanded as follows:
n ⎞
⎛ εbC (εbC) 2
(εbC)
€
PF = ΦF P0εbC⎜1−
+
− ...
⎟
2!
3!
(n + 1)!⎠
⎝
and just the first term here is significant for small εbC:
PF = ΦF P0εbC
PF is proportional to concentration at small εbC.
Fluorescence Spectroscopy
The absorption and fluorescence spectra of riboflavin.
absorption
riboflavin
fluorescence